The present invention relates generally to the fabrication of carbon-based air cathodes loaded with manganese oxide for metal air cells, and in particular, relates to the preparation of manganese oxide for such cathodes using a sol process.
Metal-air batteries produce electricity by electrochemically coupling in a cell a reactive metallic anode to an air cathode through a suitable electrolyte. As is well known in the art, an air cathode is typically a sheet like member having opposite surfaces that are exposed to the atmosphere and to an aqueous electrolyte of the cell. During operation, oxygen from air dissociates at the cathode while metal of the anode oxidizes, thereby providing a usable electric current flow through the external circuit between the anode and the cathode.
As the air cathode is extremely compact yet has essentially unlimited capacity, very high energy densities are achieved, resulting from the high volume available for the anode active material. For this reason, a metal-air cell can provide more watt-hours of electromotive force than a so-called xe2x80x9ctwo-electrode cellxe2x80x9d of similar cell size, mass and anode composition that contains both anode- and cathode-active materials inside the cell structure. Of the potential metal-air battery candidates, zinc has received the most attention because it is the most electropositive metal, which is relatively stable in aqueous and alkaline electrolytes without significant corrosion. In a zinc-air battery, the anode contains zinc and, during discharge, oxygen from the ambient air and water from the electrolyte is converted at the cathode to hydroxide, the zinc is oxidized at the anode by the hydroxide, and water and electrons are released to provide electrical energy.
Metal-air batteries such as zinc air are usually disk-like in appearance and are therfore referred to commonly as button or coin cells. These batteries are ideal power sources for small electronic devices such as hearing aids, and are disclosed in U.S. Pat. No. 5,721,065 issued Feb. 24, 1998, assigned to Rayovac Corporation, and entitled xe2x80x9cLow Mercury, High Discharge Rate Electrochemical Cellxe2x80x9d the disclosure of which is hereby incorporated by reference as if set forth in its entirety herein.
The increase in power consumption of the latest models hearing aids, such as digital hearing aids, has increased demand for high power cells. The power output of zinc air cells is mainly limited by the high polarization of air cathode that is ascribed to a slow catalytic reduction of oxygen. The air cathode used in electrochemical systems such as zinc air cells is designed to optimize the contact between the reactant and the electrolyte to maximize the reaction rate. Catalysts are commonly incorporated into air cathode structure to increase the rate of oxygen reduction. There have been many attempts over the years to find inexpensive highly active catalysts and a process for making them. However, the choice of materials is severely limited since any catalyst developed for this purpose must not only have a high activity towards oxygen reduction, but must be able to withstand a corrosive environment that would be encountered in an electrochemical cell.
Conventionally, the air cathodes used in electrochemical systems comprise many components and are typically made up of one, two or even more layers of these components. Typically, air cathodes contain an admixture of carbon, one or more catalysts, and polytetrafluorethylene (PTFE). Such a carbon-based admixture known as active layer are supported by a current collecting substrate, which usually consists of a cross-bonded screen having nickel metal strands woven therein, or a fine-mesh expanded metal screen. Finally an air diffusion layer, which usually consists of one or more pure PTFE membranes is laminated to the active layer to form a carbon-based air cathode.
It has been discovered that manganese oxide material provides a suitable catalyst for carbon-based air cathodes. For example, U.S. Pat. No. 4,433,035 entitled xe2x80x9cCatalytic Cathode Composition for Air-Depolarized Cell and Method of Preparationxe2x80x9d and U.S. Pat. No. 5,378,562 entitled xe2x80x9cMethod of Making Air Cathode Material Having Catalytically Active Manganese Compounds of Valance State+2xe2x80x9d both disclose carbon based air cathodes loaded with manganese oxide through the reduction of potassium permanganate with either carbon black or activated carbon. Similarly, patent publication WO01/37358A2, entitled xe2x80x9cCathodes for Metal Air Electrochemical Cellsxe2x80x9d discloses an admixture of silver permanganate and carbon black, wherein silver permanganate is reduced in-situ by carbon black to form a manganese oxide/silver catalyst mixture supported on carbon, which is used as cathode for oxygen reduction. However, the reaction of carbon with an oxidizing agent such as potassium permanganate predominantly results in the formation of surface oxygen compounds that make the carbon materials more hydrophilic. During operation, an active layer that is too hydrophilic might become flooded and therefore restrict gas access. Consequently, increasing the hydrophilicity of active layer might therefore decrease electrode performance. Also, such surface oxygen compounds have effects on the electrical conductivity and chemical reactivity of carbon materials used as catalyst support
French patent 2,659,075 entitled xe2x80x9csol-gel process for the preparation of manganese oxidexe2x80x9d discloses the fabrication of manganese oxide via the reduction of potassium permanganate solution with a carboxylic acid having four carbon atoms. This method produces a manganese (IV) oxide gel with fumaric acid used as the reducing agent. It has been determined that the four-carbon nature of the reducing agent yields a gel, in which the manganese oxide particles are flocculated. Furthermore, this patent does not disclose a method of preparing manganese oxide particles that are suspended in solution and suitable to combine with carbon slurry to form the supported catalyst.
From a processing and performance standpoint, it would be desirable to preserve the surface chemistry that influences the physico-chemical properties such as wettability and electrical properties of carbon materials. What is therefore needed is a more reliable process of fabricating a low polarization carbon-based air cathode loaded with manganese oxide to raise the operating voltage of metal-air cells.
In accordance with one aspect of the invention, a method is provided for producing a cathode mixture having manganese oxide particles. The method includes the steps of providing a first chemical compound having manganese, providing an organic reducing agent having one carbon atom, mixing the first chemical compound with the reducing agent to yield a manganese oxide compound, and adding the manganese oxide compound to a carbon slurry to produce a suspension of carbon slurry with suspended manganese oxide particles.
These and other aspects of the invention are not intended to define the scope of the invention for which purpose claims are provided. In the following description, reference is made to the accompanying drawings which form a part hereof, and which there is shown by way of illustration, and not limitation, preferred embodiments of the invention. Such embodiments do not define the scope of the invention and reference must therefore be made to the claims for this purpose.